How to reduce polygon count
It is impossible to exactly reduce the amount of faces of an object, but there are tricks to doing so:

1) Use the decimate modifier, and adjust the settings while watching the faces-count and the quality of the object.
Here's how this works: Go to the modifiers tab, and select decimate. Then set this modifier to un-subdivide. The modifier will show the amount of faces in the object at the bottom. Set the iterations amount, (a couple should do fine), and watch the quality of the object and the amount of faces.

2) Another thing to do: Limited Dissolve in edit mode. This won't let you specify the face-count, but you can adjust it so that you get a lower-poly result.
Do this by selecting the object, going to edit mode, and selecting mesh > cleanup > limited dissolve. Adjust the angle factor, and watch the faces. 

3) Nobody wants to do this, but go to edit mode, select certain faces and press F on the computer keyboard. Manually combine the faces until you have exactly 1000 faces (if you want), or as many as needed.

I hope these tips help you, and/or answer your question!